After the problem was solved and my address book was successfully updated to mydomain.com ,I was able to clear the autocomplete for all my users using Outlook 2010 on Windows. 

But I have 2 users using Outlook 2011 on Mac (which i am not very familiar with) and their autocomplete still show the users with mydomain.onmicrosoft.com and I would like to clear it , how can this be accomplished?